CHAPTER 3 TRANSMISSION LINES

Advantages of Suspension Insulator

a. Each unit is designed for operation voltage of about 11 kV, so
that string can be assembled by connecting several units to suit
the service voltage & weather conditions.

b. In case the line is to operate on a higher voltage in the future ,
additional units would be introduced to the same string.

c. In case of damage to one of the units, only the damaged
insulator, but not whole string, is replaced by a new one .

d. There is a decreased liability to lighting disturbances if the string
be suspended from a metallic supporting structure.

Advantages of Suspension Insulator

O can be made suitable for any voltage
level.

O If any one of the disc insulators in a
suspension string is damaged, it can be
replaced much easily.

O Mechanical stresses on the suspension
insulator is less since the line hanged on
a flexible suspension string.

O the conductors may be safe from
lightening.

Disadvantages of Suspension
Insulator

O costlier than pin and post type
insulator.

O requires more height of supporting
structure than that for pin or post
insulator to maintain same ground
clearance of current conductor.

O more spacing between conductors
should be provided.

Strain/Tension type insulator

Tension insulator are designed for handling
mechanical stresses at angle positions where
there is a change in the direction or line or at
termination of the line

Strain type

Advantages

1. For the low voltages lines, <11kV, strain insulator is
being use

1. For the high voltage transmission line, strain insulator
consists of assembly of suspension insulator.

Disadvantages

1.Not suitable to be fitted in low post
2.Hard to do maintenance



Self assessment

1.Based on suitable diagram, draw an equivalent
circuit for medium transmission line system.

2.Identify THREE methods to reduce corona’s effect.
3.Identify THREE types of insulator in overhead

transmission line
4. Based on diagram, draw the pin insulator.
5. Compare the advantages and disadvantages

suspension insulator with pin insulator.

Potential Distribution over
suspension insulator string

Refering to above, known;
C : Mutual capacitance
C1 : Shunt fitness or air fitness
V1 : Voltage negotiate first suspension insulator

unit (near to tower post)
V2 : Voltage negotiate second suspension

insulator unit.
V3 : Voltage negotiate third suspension insulator
unit (near to conductor)
E : Voltage between conductor and earth.
Take K = C1 / C or C1 = KC





example

A network of four the insulation used to hang a
conductor 33kV, three phase overhead line. Air
or bypass capacitance between each lid and
the tower is one tenth (1 / 10) of the
capacitance per unit. Calculate the voltage
across each insulator

[ans: V1 = 3764.7V , V2 = 4141.2V, V3 = 4931.8V,
V4 = 6215.5V]

• Given = 33kV Solution

• K = C1/C = 1/10 = 0.1

V2 = V1(1+K)

V3 = V1(1 + 3K + K2 )

V4 = V1 (1+6K+5K2 + K3 )

From the listed

V2 = V1(1+K)

V2 = V1 (1+0.1)

V2 = 1.1 V1

V3 = V1(1 + 3K + K2 )
V3 = V1(1+3(0.1) + 0.12 )
V3 = 1.31V1

V4 = V1 (1+6K+5K2 + K3 )
V4 = V1(1+6(0.1) + 5(0.12) + (0.13) )

V4 = 1.651V1

• Voltage between tower conductor and earth
E = V1 +V2 +V3 +V4
E = V1 +1.1V1 +1.31V1 + 1.651V1
E= 5.061V1
E = 33000/ 3 = 19050V
V1 = E /5.061

= 19050 /5.061 = 3764.7V
V2 =???
V3 = ???
V4 = ???

Network Efficiency

example

A network of four the insulation used to hang a
conductor 33kV, three phase overhead line. Air
or bypass capacitance between each lid and the
tower is one tenth (1 / 10) of the capacitance
per unit. Calculate the insulation efficiency of
the network

[ans: 76.6%]

Solution

• Network efficiency = E / nVT x 100%
= 19050V / (4)(6215.5V)x100%
=76.6%

Method of improving string efficiency

Method # 1. By Using Insulators with Larger
Discs or by Providing Each Insulator Unit with a
Metal Cap:
• Mutual capacitance greater than shunt

capacitor .
• This can be achieved by using insulators with

larger discs or providing each insulator unit
with a metal cap. The ratio K can be made 1/6
to 1/10 by this method.

Method of improving string efficiency

Method # 2. By Using Longer Cross-Arms:
• The ratio of shunt capacitance to mutual capacitance, K

can alternatively be reduced by using longer cross-
arms so that the horizontal distance from line support
(pole or tower) is increased thereby decreasing the
shunt capacitance.
• But the limitations of cost and mechanical strength of
line supports do not allow the cross-arms to be too
long and it has been found that in practice it is not
possible to obtain the value of K less than 0.1.

Method of improving string efficiency

Method # 3. By Capacitance Grading:
• It is seen that non-uniform distribution of voltage across an

insulator string is due to leakage current from the insulator
pin to the supporting structure.
• This current cannot be eliminated. However, it is possible
that discs of different capacities are used such that the
product of their capacitive reactance and the current
flowing through the respective unit is same.
• This can be achieved by grading the mutual capacitance of
the insulator units i.e., by having lower units of more
capacitance—maximum at the line unit and minimum at
the top unit, nearest to the cross-arm.. So this method is
not used in practice below 200 kV.

Method of improving string efficiency

Method # 4. By Static Shielding/guard ring:
• In case of capacitance grading, insulator units of

different capacitances are used so that the flow
of different currents through the respective units
produce equal voltage drop.
• In static shielding, pin to supporting structure
charging currents are exactly cancelled so that
the same current flows through the identical
insulator units and produce equal voltage drops
across each insulator unit.



Improve Insulator Distribution

1.Use of long cross arm
2.Capacitance grading
3.Use of grading rings

TYPES OF TRANSMISSION TOWERS

Single circuit tower

Double circuit tower

Quad circuit tower

DIFFERENT DESIGN OF TRANSMISSION TOWERS
LATTICE TOWERS

275kV 132kV
132kV

TRANSMISSION TOWERS IN RAWANG 500kV TOWERS NEAR PMU BKT TAREK 500/275kV

275kV 275kV
275kV 132kV
33kV132kV
132kV
TOWERS CROSSING PLUS HIGHWAY NEAR TG MALIM
TRANSMISSION TOWERS IN KELANG

ILLUSTRATION OF R-O-W REQUIREMENTS FOR
TNB

DIFFERENT DESIGN OF TRANSMISSION TOWERS
MONOPOLE TOWERS

132kV TERMINAL TOWER AT PMU SEGAMBUT 132kV MONOPOLE TOWER IN BUKIT PANTAI

132kV MONOPOLE TOWER
IN SRI HARTAMAS

33kV OVERHEAD DISTRIBUTION LINES

33kV TOWERS IN TAMAN TASEK TITIWANGSA 33kV TOWERS IN TMN NIRWANA, AMPANG

33kV TOWERS IN DENGKIL 33kV TOWER IN KG AMPANG CAMPURAN